Ever since the break up of AT&T, companies have tried to save money on their telephone bills. First came changing your long distance carrier. You could get rates that were half or even lower then what AT&T charged. Eventually dozens of companies got involved in reselling long distance to the point where rates have gone to almost $.01 per minute.
Later on came changing your local carrier. Many companies resold the local exchange carrier's lines at a discount. Service was not as good, but the discounts were compelling for many companies.
Now the hot new thing is VoIP. You can get rid of the local phone company all together. You can get your dial tone from your cable company or though your DSL line.
Businesses can get rid of the local carrier too. High speed data lines allow businesses so get their dial tones from their ISP. The discounts are again very compelling. Be very careful, the service you get from some VoIP carriers is very poor. It's not sound quality, it's the up time. Make sure to deal with an established, reputable company and don't let promised savings blind you.
Talk with your telephone system vendor. They probably have relationships with several VoIP carriers. Get references from companies in your neighborhood. That will really help you gauge the reliablity of the service.
Keep some back up service on regular copper lines. If your VoIP service goes down, you can have your calls forwarded to your back up service. It's well worth the extra costs. http://www.teleco4.com